It is known to etch the printing surface of each printing plate used in color reproduction printing to provide circular or rectilinear patterns composed of a various number of wells or cells per square inch. The wells may all have the same surface area with varying depths to control the tonal shading or coloration. Alternatively, the depth of the wells may remain constant and the surface of the wells be varied to achieve the same differences in shading or tone and color reproduction. This style of printing is conventional and disclosed in U.S. Pat. No. 923,799.
This gravure type printing conventionally has well depths which vary from 35 microns for solid colors to 2 microns for the light color tones or tints. In some cases, greater depths are required. Wear along the printing surface rapidly changes the volume in the extremely shallow wells, thereby causing well known disadvantages. As known, each of the wells is separated by a bridge defined by a solid printing surface section. At the areas of contact between the paper and the bridge surface, there is no ink transfer. The small size of the individual wells, the bridge surfaces, the wear of the printing plate, and the inability of capillary action to adequately remove all of the ink from the well during the printing process wherein ink is transmitted from the well to the paper, each contributes to a discontinuous and irregular printing result.
Conventionally, a color image is reproduced by superpositioning imprints from a plurality of screen pattern, color separation plates. The multi-color photo engraving practice is often impaired by objectionable interference or moire patterns. These appear as objectionable grid patterns having a distinct geometric design. Furthermore, if the printed or reproduced image is magnified, a reproduction of the pattern of the printing screen is seen to exist. One prior art attempt to obviate this problem involves the use of a plurality of photoengravers' screens being rotated through different angles relative to a horizontal base line for several exposures onto the color separation plates used in the multi-color photoengraving printing process. That is, the separate screens are disposed, one on top of the other, and rotated to different angles with respect to a horizontal base line for the several exposures so that the lines of the screen patterns fall at different angles with respect to the base line. Each plate, or screen, used is for a different color. This prior art procedure is fairly common. However, the presence of a screen pattern in the reproduced image is still present.
The prior art associated with this particular field is exemplified by the U.S. Pat. Nos. 521,659, 1,872,943, 2,206,054, 2,360,587, 2,757,087, 2,768,577, 2,798,428, 3,288,059 and 3,450,044. All of these patentees describe printing procedures which are related to the production of printing screens and the use of same during the printing of a reproduced image. While these set forth conventional multi-screening procedures to reproduce both black and white and color reproductions, none of these prior art procedures has resulted in the production of a substantially continuous tone printing effect wherein the printed or reproduced image includes no trace of a screen or grid pattern particularly under magnified conditions.
The U.S. Pat. Nos. 2,200,285, 2,621,245 and 3,663,221 each relates to the use of separate gratings to transmit separate colors to an image screen. However, none address the specific problem solved with the instant invention.